In recent years, a compact and slim image pickup device has been mounted on a portable terminal, which is a compact and slim electronic device, including a cellular mobile phone and PDA (Personal Digital Assistant). This has permitted visual information as well as audio information to be exchanged between distant places. A solid state image sensor such as a CCD (Charge Coupled Device) type image sensor and CMOS (Complementary Metal Oxide Semiconductor) type image sensor is employed as an image sensor used in the image pickup device. Further, km the purpose of achieving low-cost production, a plastic lens that can be mass-produced at a lower cost is utilized as an imaging lens for forming a subject image on an image sensor.
One of the methods for producing a lens and optical system is a replica method wherein a plurality of lenses can be formed simultaneously on one parallel plane surface plate. The Patent Document 1 proposes an imaging lens wherein such a replica method is used to form a diffraction surface and refraction surface simultaneously on the parallel plane surface plate, whereby chromatic aberration is corrected.
The Patent Document 2 discloses an example of an imaging lens used in an image pickup device (camera module) incorporated in a portable terminal. The imaging lens of Patent Document 2 is made of one double-convex lens. This has succeeded in reducing the overall optical length. However, the double-convex lens cannot easily reduce a longitudinal chromatic aberration. Moreover, a field curvature is large on the periphery. In addition, when the image side surface is convex toward the image side, an inflection point must be provided on the image side surface so as to allow the peripheral portion and the paraxial portion to have different powers, so that the focal distance of the light beam through the peripheral portion of the lens has a larger focal length and the image plane is adjusted. In this case, however, astigmatism occurs close to the mid-band area wherein the inflection point is provided, with the result that the MTF (Modulation Transfer Function) is seriously deteriorated.
For example, the imaging lens of Patent Document 3 is composed of a meniscus lens convex toward the image side. However, the configuration of this imaging lens has a large negative distortion, and that distortion is difficult to reduce.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-323365
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-10750
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-296473